Amplifier circuit for optical disk drive

ABSTRACT

An operational amplifier including a pair of differential input transistors is provided with a feedback resistor switching circuit and a variable current source. When a loaded optical disk is a DVD-ROM, the feedback resistor switching circuit reduces a gain of the operational amplifier and the variable current source selects a large current (of, for example, 1 mA) as a common bias current supplied to the differential input transistors. When the loaded optical disk is a DVD-RAM with low reflectance and low recording/reproducing speed, the feedback resistor switching circuit enhances the gain of the operational amplifier and the variable current source selects a small current (of, for example, 0.5 mA) as the common bias current.

BACKGROUND OF THE INVENTION

The present invention relates to an amplifier circuit, for use in anoptical disk drive, for converting a photoelectric current into avoltage.

Today, a variety of media types of optical disks, such as CDs (compactdisks) and DVDs (digital versatile disks), have been practically used.The CDs can be roughly classified into read only disks and writabledisks. The read only disks include CD-DAs for digital audio, CD-ROMsmainly used for recording computer data and the like. The writable disksinclude CD-Rs that are once writable, CD-RWs that are writable anderasable and the like. The DVDs can be also roughly classified into readonly disks and writable disks. The read only DVDs include DVD-ROMs andthe like, and the writable DVDs include DVD-Rs that are once writable,DVD-RAMs that are writable and erasable and the like. In considerationof such a situation, a recent optical disk drive has become applicableto a plurality of media types.

In general, an optical disk drive irradiates an optical disk loadedtherein with a laser beam so as to reproduce data by detecting reflectedlight. For example, an infrared laser beam is used for a CD and a redlaser beam is used for a DVD. A light receiving unit of the optical diskdrive includes a photodiode for generating a photoelectric current inaccordance with the reflected light from the optical disk, and thephotoelectric current is converted into a voltage by an amplifiercircuit (i.e., a current-voltage conversion circuit).

Japanese Laid-Open Patent Publication No. 10-340526 discloses atechnique to discriminate a media type of an optical disk by detectingreflectance of the optical disk on the basis of a peak level of a focuserror signal. This publication further discloses an optical disk drivethat selects any of a plurality of amplifier circuits having differentgains or controls a power of the laser beam in accordance with thediscriminated media type. Specifically, for example, when a CD-DA/ROMand a CD-RW are compared with each other, the reflectance of the CD-RWis as low as approximately ¼ of that of the CD-DA/ROM due to theirdifference in recording principle. Accordingly, in reproducing data froma CD-RW, an amplifier circuit with a large gain is selected or the powerof the laser beam is increased, so that an output voltage level can beconstant regardless of media type.

Japanese Laid-Open Patent Publication No. 2000-293933 discloses anoptical disk drive that can control a gain of a single amplifier circuiton the basis of a discriminated media type. Specifically, for a DVD-RAMwith low reflectance, a power of the laser beam is increased as comparedwith that for a DVD-ROM and a resistor with a large resistance value isselected as a feedback resistor of an operational amplifier included inthe amplifier circuit, so that the gain of the amplifier circuit can beenhanced. Thus, an output level of the amplifier circuit can be constantregardless of media type.

On the other hand, a reproducing speed for DVD-ROMs is increasing. Forexample, for attaining 16× reproducing speed as compared with standardone, an amplifier circuit is required to have a frequency band as wideas 72 MHz (1 dB down) for DVD-ROMs. A recording/reproducing speed forDVD-RAMs is not demanded to be as high as the reproducing speed forDVD-ROMs, but there is a strong demand for lowering noise inrecording/reproducing of DVD-RAMs.

However, a large bias current is conventionally supplied commonly to apair of differential input transistors of an operational amplifierincluded in an amplifier circuit for converting a photoelectric currentinto a voltage, so as to uniformly attain a wide band characteristicregardless of media type. Accordingly, shot noise is caused in each ofthe pair of differential input transistors to the same extent in loadinga DVD-ROM as in loading a DVD-RAM. Moreover, since a gain of theamplifier circuit is enhanced in loading a DVD-RAM, an output noise isincreased. Also, this means that power is wastefully consumed in loadinga DVD-RAM.

SUMMARY OF THE INVENTION

An object of the present invention is reducing noise and powerconsumption when an optical disk that does not need an amplifier circuitwith a wide frequency band is loaded in an optical disk drive.

In order to achieve the object, the amplifier circuit of this invention,for use in an optical disk drive capable of coping with optical disks ofa plurality of media types, is an amplifier circuit for converting, intoa voltage, a photoelectric current generated by a photodiode inaccordance with reflected light from a loaded optical disk, and includesan operational amplifier including a pair of differential inputtransistors; and a variable current source for selecting, in accordancewith a discriminated media type of the loaded optical disk, a largecurrent when the loaded optical disk needs a wide frequency band(namely, when the optical disk is a read only disk such as a DVD-ROM ora CD-DA/ROM) and a small current when the loaded optical disk does notneed a wide frequency band (namely, when the optical disk is a writabledisk such as a DVD-R/RAM or a CD-R/RW), as a common bias currentsupplied to the pair of differential input transistors.

According to the invention, the common bias current supplied to the pairof differential input transistors of the amplifier circuit is changed inaccordance with the media type of the loaded optical disk. Therefore,noise and power consumption can be reduced when the amplifier circuitworks as an amplifier circuit for, for example, a DVD-RAMrecording/reproducing apparatus while keeping a frequency characteristicnecessary as an amplifier circuit for a DVD-ROM reproducing apparatus.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram for showing an exemplified structure of anoptical disk drive using an amplifier circuit according to theinvention;

FIG. 2 is a circuit diagram for showing, together with a photodiode, anexemplified configuration of the amplifier circuit of this inventionincluded in an optical pickup of FIG. 1;

FIG. 3 is an output level-frequency characteristic diagram of theamplifier circuit of FIG. 2; and

FIG. 4 is an output noise-frequency characteristic diagram of theamplifier circuit of FIG. 2.

DETAILED DESCRIPTION OF THE INVENTION

Now, a preferred embodiment of the invention will be described in detailwith reference to the accompanying drawings.

FIG. 1 shows an exemplified structure of an optical disk drive using anamplifier circuit according to this invention. This optical disk drivecan be operated as a high speed DVD-ROM reproducing apparatus and a lowspeed DVD-RAM recording/reproducing apparatus. In FIG. 1, a referencenumeral 1 denotes an optical disk (a DVD-ROM or a DVD-RAM) loaded in theoptical disk drive, a reference numeral 2 denotes a spindle motor, areference numeral 3 denotes an optical pickup, a reference numeral 4denotes a front end processor (FEP), a reference numeral 5 denotes aservo controller, a reference numeral 6 denotes a digital signalprocessor (DSP) and a reference numeral 7 denotes a system controller.The optical pickup 3 irradiates the optical disk 1 with a laser beam anddetects reflected light from the optical disk 1. The FEP 4 processes anoutput from the optical pickup 3 through an analog operation, filteringand the like, and supplies disk discrimination signals S(ROM) andS(RAM), corresponding to the media type of the optical disk 1, to theoptical pickup 3. It is herein assumed that the media type isdiscriminated on the basis of, for example, the level of the reflectedlight from the optical disk 1, so that the signals S(ROM) and S(RAM) canbe respectively “H” and “L” when the optical disk 1 is a DVD-ROM and thesignals S(ROM) and S(RAM) can be respectively “L” and “H” when theoptical disk 1 is a DVD-RAM. The servo controller 5 performs, inaddition to a servo control of the spindle motor 2, a focus servocontrol, a tracking servo control and a laser power control of theoptical pickup 3. The DSP 6 performs digital signal processing such aserror correction and signal reproducing processing. The systemcontroller 7 controls the entire operation of the optical disk drive.

FIG. 2 shows, together with a photodiode 10, an exemplifiedconfiguration of the amplifier circuit of this invention included in theoptical pickup 3 of FIG. 1. This amplifier circuit, which converts aphotoelectric current Iin generated by the photodiode 10 into a voltageVout, includes an operational amplifier 20, a feedback resistorswitching circuit 30 and a variable current source 40, and forms,together with the photodiode 10, one opto-electric conversion IC, thatis, the so-called OEIC.

In the operational amplifier 20 functioning as a trans-impedanceamplifier receiving the current Iin and outputting the voltage Vout,reference numerals 21, 22, 25 and 26 respectively denote first throughfourth NPN transistors, reference numerals 23 and 24 respectively denotefirst and second PNP transistors, a reference numeral 27 denotes a firstresistor, Vcc denotes a supply voltage (of, for example, 5 V), Vrefdenotes a reference voltage (of, for example, 2.2 V), Vb denotes a biasvoltage and Ib denotes a bias current. The first and second NPNtransistors 21 and 22 commonly receive, from the variable current source40, the bias current Ib at their emitters, so as to together work as apair of differential input transistors. A base of the first NPNtransistor 21 is connected to the reference voltage Vref and a base ofthe second NPN transistor 22 is connected to a cathode of the photodiode10. The third NPN transistor 25 functions as an emitter follower foroutputting the voltage Vout.

The feedback resistor switching circuit 30 is disposed between anemitter of the third NPN transistor 25 and the base of the second NPNtransistor 22. In the feedback resistor switching circuit 30, referencenumerals 31, 33, 34 and 36 respectively denote second through fifthresistors and reference numerals 32 and 35 respectively denote fifth andsixth NPN transistors. It is herein assumed that the second and fourthresistors 31 and 34 respectively have resistance values of 5 kΩ and 20kΩ. In the case where a DVD-ROM is loaded as the optical disk 1, thesignal S(ROM) is “H” and hence the fifth NPN transistor 32 is turned on,resulting in selecting the second resistor 31 with a resistance value of5 kΩ as a feedback resistor of the operational amplifier 20. On theother hand, in the case where a DVD-RAM is loaded as the optical disk 1,the signal S(RAM) is “H” and hence the sixth NPN transistor 34 is turnedon, resulting in selecting the fourth resistor 34 with a resistancevalue of 20 kΩ as the feedback resistor of the operational amplifier 20.Therefore, a gain of the amplifier circuit obtained in loading a DVD-RAMis four times as large as that obtained in loading a DVD-ROM.

In the variable current source 40 for supplying the bias current Ib,reference numerals 41 and 44 respectively denote seventh and eighth NPNtransistors and reference numerals 42 and 43 respectively denote sixthand seventh resistors. It is herein assumed that the bias voltage Vb is0.9 V, that the base-emitter voltage of the seventh NPN transistor 41 is0.7 V, and that both the sixth and seventh resistors 42 and 43 have aresistance value of 200 Ω. In the case where a DVD-ROM is loaded as theoptical disk 1, the eighth NPN transistor 44 is turned on because thesignal S(ROM) is “H”, and hence, the bias current Ib is (0.9 V-0.7V)/200 Ω=1 mA. On the other hand, in the case where a DVD-RAM is loadedas the optical disk 1, the eighth NPN transistor 44 is turned offbecause the signal S(ROM) is “L”, and hence, the bias current Ib is (0.9V-0.7)/(200 Ω+200 Ω)=0.5 mA.

Results of simulation carried out on the amplifier circuit of FIG. 2 areshown in FIGS. 3 and 4. FIG. 3 shows an example of an outputlevel-frequency characteristic of the amplifier circuit, and FIG. 4shows an example of an output noise-frequency characteristic of theamplifier circuit. According to FIG. 3, the output level in a highfrequency region obtained when a DVD-RAM is loaded (namely, when thebias current Ib is 0.5 mA) is slightly lower than that obtained when aDVD-ROM is loaded (namely, when the bias current Ib is 1 mA). On theother hand, according to FIG. 4, the output noise in a frequency regionbelow 200 MHz obtained when a DVD-RAM is loaded (namely, when the biascurrent Ib is 0.5 mA) is reduced. This is probably because shot noisecaused in the bases of the pair of differential input transistors 21 and22 included in the operational amplifier 20 is reduced.

In this manner, according to the amplifier circuit of FIG. 2, the biascurrent Ib supplied to the pair of differential input transistorsincluded in the operational amplifier 20 is changed in accordance withthe discriminated media type of the loaded optical disk 1, so that noiseand power consumption can be reduced when the amplifier circuit works asan amplifier circuit for a DVD-RAM recording/reproducing apparatus whilekeeping a frequency characteristic necessary as an amplifier circuit fora DVD-ROM reproducing apparatus. In addition, the amplifier circuit ofFIG. 2 can switch the feedback resistor and the bias current of thesingle operational amplifier 20, and hence, the chip area can be reducedas compared with the structure in which any of a plurality of amplifiercircuits having different gains is selected. Accordingly, this amplifiercircuit of FIG. 2 is suitably used in an OEIC.

The present invention is applicable to an arbitrary optical disk drivecapable of coping with a variety of types of media including DVDs andCDs. The invention is also applicable to an optical disk of the nextgeneration that is expected to realize high density recording by using ablue laser beam.

For example, in the case where the invention is applied to CDs, theaforementioned control for a DVD-ROM corresponds to control for a CD-ROMand the aforementioned control for a DVD-RAM corresponds to control fora CD-RW. The numbers of feedback resistors and bias currents to beswitched in the operational amplifier 20 can be arbitrarily determined.In application to a DVD-RAM or a CD-RW, the feedback resistor and thebias current of the operational amplifier 20 may be switched between arecording operation and a reproducing operation. Furthermore, a power oflaser beam may be controlled together.

Also, the media type of the optical disk 1 can be arbitrarilydiscriminated. For example, disk identification information previouslyrecorded in an inner circumferential portion of the loaded optical disk1 may be read so that, for example, the DSP 6 can supply the diskdiscrimination signal in accordance with this information. Needless tosay, any of various discrimination methods described in JapaneseLaid-Open Patent Publication Nos. 10-340526 and 2000-293933 may beemployed.

What is claimed is:
 1. An amplifier circuit, for use in an optical diskdrive capable of coping with optical disks of a plurality of mediatypes, for converting, into a voltage, a photoelectric current generatedby a photodiode in accordance with reflected light from a loaded opticaldisk, comprising: an operational amplifier including a pair ofdifferential input transistors; and a variable current source forselecting, in accordance with a discriminated media type of said loadedoptical disk, a large current when said loaded optical disk needs a widefrequency band and a small current when said loaded optical disk doesnot need a wide frequency band, as a common bias current supplied tosaid pair of differential input transistors.
 2. The amplifier circuit ofclaim 1, further comprising a feedback resistor switching circuit forselecting, in accordance with the discriminated media type of saidloaded optical disk, a resistor with a small resistance value when saidloaded optical disk has high reflectance and a resistor with a largeresistance value when said loaded optical disk has low reflectance, as afeedback resistor of said operational amplifier.
 3. The amplifiercircuit of claim 2, wherein, when said loaded optical disk is a readonly disk, said resistor with the small resistance value is selected bysaid feedback resistor switching circuit as said feedback resistor ofsaid operational amplifier and said large current is selected by saidvariable current source as said common bias current supplied to saidpair of differential input transistors, and when said loaded opticaldisk is a writable disk, said resistor with the large resistance valueis selected by said feedback resistor switching circuit as said feedbackresistor of said operational amplifier and said small current isselected by said variable current source as said common bias currentsupplied to said pair of differential input transistors.
 4. Theamplifier circuit of claim 3, wherein said read only disk is a read onlyDVD such as a DVD-ROM and said writable disk is a writable DVD such as aDVD-R or a DVD-RAM.
 5. The amplifier circuit of claim 3, wherein saidread only disk is a read only CD such as a CD-DA or a CD-ROM and saidwritable disk is a writable CD such as a CD-R or a CD-RW.